why do people get mean when they're drunk?

I'm just going to use my mum for an example. I never have had a close relationship with her, it's not like shes abusive physically or mentally - its more that she gives critism all the time instead of emotional support - to be fair this makes sense because i am 18 and no longer a child but even as a child, compared to my other friends and their mothers - it's almost as if she just wants me to stop crying not bc of my negative emotions but because well, im crying if that makes sense. idk i just never felt genuine to me.
But whenever she gets drunk, she just makes everyone in the family irritated because she acts like a child and keeps on repeating things. Basically isnt thinking straight and saying stupid stuff - which is understanding bc thats a common thing when ur drunk.
But for example today, I was washing my face and she came into the bathroom to brush her teeth. For some context, I have acne and it's taken me 4 years and im finally seeing massive improvements because I am much more educated about skincare and i finally got given a prescription and advice from a dermatologist. I've gotten compliemts and people told me that they see my skin is getting better - which makes me very happy bc at least i know its working and its not in my head.
I was minding my business but my mum kept saying stuff like "i dont know why your doing that to your face, its going to ruin your skin" and "when you're my age, you're going to be covered in wrinkles" and "your acne is getting worse bc ur doing this" and "I'm always going to look better than you bc you do this to your skin". At this point I was angry. I told her that google and dermatologists know much more than her, and its annoying me that she thinks shes smarter than everyone else. I told her to go away and that she doesnt know what shes talking about because she as educated in the subject.
Then she started going on about how when she dies, I will regret not taking her advice. How when she dies, no one will tell me what truly to do. She kept saying that she was "sorry". She was crying and i know i should feel bad but the reason why i feel more anger than sadness was because she was being sarcastic. "I'm so sorry that google and dermatologists know more about skin that me" and "Look at me, I am sorry. so so sorry".
She slammed the door and left. I know i should feel bad as shes my mother and i made her cry. I'm not bothered by the fact that she's calling me ugly, well i am but i'm more bothered at the fac that she treats me like im dumb and that she pretends knows much more than a professional by telling me insults. She's always like this when shes drunk and alot of the time it ends in tears bc she couldnt handle what i said back. She always brings a scenario up when she dies, and it just drives me crazy more.
Why does she get like this?